Control of drive configurations or characteristics of all-terrain or other types of utility vehicles can be unique to such vehicles. Such vehicles are often capable of being driven over uneven or hilly terrain. Such vehicles are often capable of encountering soft, loose or slippery soils as well as snow and/or ice.
A driver of such a vehicle typically operates the vehicles from a somewhat centrally located seat location using handlebars to control the steering of the vehicle, typically accomplished by turning the front wheels with respect to the body of the vehicle.
Optimum drive characteristics for these vehicles often vary from rear wheel drive, front and rear wheel drive and all wheel drive. In a typical rear wheel drive configuration, only the rear wheels propel the vehicle. The rear wheels may be rotationally coupled allowing both rear wheels to provide traction. In front and rear wheel drive configuration (sometimes referred simply as four wheel drive), the rear wheels and at least one of the front wheels provide traction. In all wheel drive configuration, the rear wheels and both front wheels provide traction.
The drive configuration is fixed for some prior art systems. That is, the drive configuration can not be changed, e.g., from rear wheel drive to all wheel drive. In other prior art systems, the drive configuration can be selected by the operator but only in certain circumstances, e.g., such as when the vehicle is stationary.
Care should be taken in choosing a drive configuration and, in particular, in changing between drive configurations. Selecting or changing from one drive configuration to another drive configuration while the vehicle is in motion may affect the steering and/or handling of the vehicle and could result in loss of control of the vehicle.
One prior art system employed to engage front wheel drive (for a base rear wheel drive vehicle) is to employ engagement dogs, or splines. A limitation with this type of drive configuration is the inability to engage or disengage “on the fly,” i.e., while the vehicle is in motion. Thus, it is typically not useful in a system that engages only when wheel slip is detected. An operator, or driver, of a vehicle using a system based upon this technique generally must anticipate driving conditions to be encountered and choose between greater traction capability or steering effort and handling. The same problem is true for engagement dogs used to control differentiation between the left and right front wheels. Additionally, this type of engagement is “all or nothing,” i.e., the engagement typically can not be modulated like a clutch that is capable of slipping.
Over-running clutches have also been used on vehicles of this type. A disadvantage of an over-running clutch is that these systems generally have a front to rear gear ratio other than one, e.g., −0.83:1, depending on the particular vehicle. This front to rear ratio is generally used due to the need to prevent engagement of four wheel drive on surfaces having good traction during turns. This ratio difference can result in sudden engagement of front wheel drive under some circumstances, as well as ultimate traction, as the front and rear wheels fight each with different rotational rates when the front wheel drive is engaged. Further, the front wheels generally can not be use for engine braking, severely limiting the vehicle's capability in steep downhill terrain.
Another method utilized employs a differential mechanism that uses differential cams and a roller clutch to engage/disengage the front wheel drive. This type of system does not allow for computerized automatic engagement of front wheel drive and will usually not supply significant power to the tractive wheel if the other wheel is completely non-tractive. Further, tuning of this type of system is fundamental to the design and usually can not accept user or computer input.
Another traditional method of traction control uses silicon viscous technology to apply linear force to a clutch pack in response to differences in speed between the left and right front wheels. Usually, this method can not be tuned as a function of vehicle speed and may compromise high-speed handling with low-speed capability. Further, this method usually can not be disengaged, even in two-wheel (rear wheel) drive mode.
Traditional automotive methods, such as silicon viscous coupling between the transfer case and the front drive are generally not desirable because they usually do not exhibit safe braking characteristics on an all-terrain or utility vehicle. All-terrain or utility vehicles usually have the capability brake the rear wheels independent of the front wheels. During application of the rear brakes, if rear wheel lock-up occurs, a difference in front wheel speed to rear wheel speed would exist. A silicon viscous front wheel drive coupling system will attempt to limit that difference. Engagement could cause rotation of the front wheels to approach the rotation of the rear wheels, but only after a delay. This delay can unexpectedly try to pitch the driver over the handle-bars and is, thus, an unsafe condition.
Another traditional automotive technique is to employ a Torsen® style limited slip device to act between the left and right front wheels. However, this type of system generally can not be automatically or manually controlled and generally will not supply significant power to a tractive wheel if the other wheel is completely non-tractive, e.g., in the air or on ice.
Another traditional technique is to employ a “limited slip” mechanism between the left and right wheels. Since all-terrain vehicles generally don't have power steering to overcome the resulting increase in steering effort, steering effort can become unacceptably high. Further, the amount of engine torque that can be transmitted to only one wheel is severely limited, reducing off-road driving capability.
Still another prior art technique uses fly-weights that spin in accordance with the difference in left and right wheel rotational speed. At some preset speed, the fly weights cause the engagement of a locking mechanism. This type of system can have a dangerous handlebar jerk and poor handling upon engagement when used with a four wheel drive vehicle.